Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in what is called a projection exposure apparatus, which includes an illumination device and a projection lens. The image of a mask (=reticle) illuminated by way of the illumination device is in this case projected by way of the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In this case, in practice there is a desire to monitor parameters characteristic of the patterned wafer, e.g. the CD value or the layer thickness. Particularly in so-called “multi-patterning” methods for undershooting the resolution limit of the optical system with structures produced on the wafer in a plurality of lithography steps, a large number of process parameters have to be monitored. The so-called overlay is often of particular importance here.
When determining such parameters it is known, inter alia, to produce auxiliary structures in the form of suitable marker regions in particular in edge regions of the wafer elements respectively produced, in order to carry out, on the basis of the auxiliary structures, a diffraction based determination of the respective relevant parameters in a scatterometric setup.
One issue that occurs in practice here is that the parameter values determined on the basis of such auxiliary structures do not necessarily represent the actual behavior of the used structures contained on the wafer, which may be attributable e.g. to an inadequate correlation between used structure and auxiliary structure and/or a large distance between these structures. A further issue that occurs in practice results occasionally from the comparatively high number of auxiliary structures involved, the progressive evaluation of which would be accompanied by a significant impediment of the throughput of the lithography method.
Furthermore, the determination of a plurality of relevant parameters within a used structure of possibly complex construction on a wafer also poses a demanding challenge insofar as the relevant parameters, under certain circumstances, can be determined simultaneously in a single measurement setup only with difficulty. This is the case if the parameters are linearly dependent in the sense that certain combinations of these values lie in the kernel (i.e. eigenvectors with respect to the eigen-value 0) of the covariance matrix describing the issue. This case often occurs in the event of a deviation between measured and simulated values that is detected in a diffraction based determination, with the result that it is not possible to decide un-ambiguously what parameter variation causes the deviation.
With respect to the prior art, reference is made merely by way of example to US 2006/0274325 A1, U.S. Pat. No. 8,339,595 B2, U.S. Pat. No. 8,670,118 B2 and US 2012/0224176 A1, US 2003/0219153 A1, US 2009/0037134 A1 and U.S. Pat. No. 7,916,286 B2.